On February 18, 1930 a 24 year old self-taught astronomer
sat concentrating through the eyepiece of the Blink-Comparator
at the Lowell Observatory. Clyde Tombaugh was searching for something
special, a dim little needle-in-the-haystack that had eluded
the keen eye of the very founder of the observatory, the esteemed
Percival Lowell. To find it would make history. It would also
prove Lowell's calculations that predicted a ninth planet perturbing
the orbits of Uranus and Neptune. Clyde Tombaugh was pursuing
the elusive Planet X.

Back
and forth he flipped the shutter of the Blink-Comparator, a machine
that presented his eye with two photographs of the identical
field of stars taken several weeks apart. Clyde had been at this
nearly a year, seeing nothing moving in the eyepiece. Suddenly,
there it was, just as Lowell had predicted and searched for in
vain for eight years himself. A faint speck jumped between the
stars. Only a moving planet would do this, and this one was right
where it should be...or so it seemed.

On March 13, 1930, the birthday of Percival Lowell, the Lowell
Observatory announced the discovery of Planet X. Some, including
his widow, thought the proof of the decades old prediction justified
naming the planet after Lowell. But planets were traditionally
named for characters in Greek and Roman mythology. So Planet
X became Pluto, the name suggested by Venetia Burney, an 11 year
old school girl in Oxford, England. Pluto, god of the underworld,
ruling in the bleak darkness of an orbit that takes 248 years
to complete. It was a name that everyone could agree on, especially
since the initials for Pluto are PL, the same as Percival Lowell.

This would be the happy end of the story, except for an increasing
number of oddities that have been accumulating over the last
sixty plus years. For one thing, we now know there never was
a Planet X. The calculations were wrong.

How's that? The slight shift in the orbit of Uranus that astronomers
couldn't fully explain, even with the discovery of Neptune in
1846, disappeared when careful measurements were made by the
interplanetary spacecraft sent to the outer planets. Almost as
strange, astronomers first assessed Pluto to be much larger and
massive than later observations justified. It turns out that
those observations proved that Pluto is smaller than our own
moon and has a mass just two tenths of one percent of the Earth.
That's just a fraction of what would be needed to account for
the presumed motion of Uranus that started astronomers looking
for a ninth planet in the first place.

What's more, Pluto's orbit around the Sun is tilted compared
to the other planets. Mercury, Venus, Earth, Mars, Jupiter, Saturn,
Uranus and Neptune line up within a degree or two of a flat plane
through the sun. Their orbits are nearly circular. Pluto's path
is tilted 17 degrees to that plane and is so elliptical that
for the 20 years from 1979 to 1999 Pluto has actually been the
8th planet of the Solar System, closer to the Sun than Neptune.

Some astronomers are starting to take a skeptical view of
little Pluto, wondering if what Clyde Tombaugh really discovered
was a large comet instead of a small planet. The composition
of Pluto is a core of rock and ice with a surface of methane
ice and a slight atmosphere of nitrogen, methane and carbon monoxide.
If Pluto's orbit was even more elliptical, it would enter the
inner solar system and shoot out a comet-like tail as the water
and gasses boil off from the heat of the sun.

Then there is the matter of Pluto's moon, Charon. Charon is
about half the size of Pluto itself and named, appropriately,
after the mythological character who ferried the dead across
the River Styx and into the underworld of Pluto. Pluto and Charon
are almost a double planet system. They dance around each other,
all the while showing exactly the same faces. Charon hangs motionless
in the Pluto sky, and vice-versa.

There is a move afoot in astronomy to "demote" Pluto
to a "minor planet" and consider it something of a
large asteroid that was misclassified in the excitement of being
found just where a planet was mistakenly expected. But...not
so fast. The whole definition of what is a planet is somewhat
murky to begin with. What scientists agree on is that a planet
must orbit the Sun and be large enough that its own gravity pulls
it into a spherical shape. Pluto easily meets that criteria.
Besides, there is no definition as to composition. Jupiter, Saturn,
Neptune and Uranus are gas giants. Mercury, Venus, Earth and
Mars are rocky. Why not an icy-rocky planet?

Perhaps
the best idea is to get out there and see what Pluto and Charon
really look like and what is going on at the edge of the Solar
System. After all, Pluto is the only planet missed on the Voyager
grand tour of the Solar System. NASA has long had plans for a
mission to Pluto. The original concept was the Pluto-Kuiper Express,
which was later changed to New Horizons. That mission successfully
launched on January 19, 2006 and is on its way to Pluto right
now. The Pluto mission hopes to map the planet and its moon,
and also the Kuiper Belt, a region of minor icy planets that
orbit the sun in almost complete darkness. Planet 9 or Minor
Planet 10,000? That is the question.

My vote is to stick with the lineup that includes Pluto and
leave the door open for yet a planet 10, 11, or ... who knows?
We're just starting to really explore space, and who can say
what fortuituous discoveries await us just beyond our present
field of view. Considering the happy accident of discovering
a planet exactly where one miscalculated it to be suggests there
is so much more that we just haven't thought to look for yet.
When we do, we might just find it!

Note: Sadly, on August 24, 2006,
the International Astronomical Union decided to recategorize
Pluto as a "dwarf planet." Only a minority of the members
participated in this vote.

Books of Interest:

Pluto and Charon by Alan Alan Stern (Editor), David
J. Tholen (Editor), S. Alan Stern (Editor). This 756 page book
has an extensive collection of scientific information from a
description of the discoveries of Pluto and Charon to composition,
internal structure and thermal evolution to chemical models of
Pluto's atmosphere and interaction with the solar wind. Read
the extensive table of contents for more details.

Pluto and Charon: Ice Worlds on the Ragged Edge of the
Solar System by Alan Alan Stern, Jacqueline Mitton. Now one
of the world's leading experts on Pluto shares the most current
information on the last exciting frontier for astronomers in
one lively, illustration-packed volume. Based on the latest information
gleaned from the new technology of groundbased astronomical instrumentation
and spacecraft explorations after the 1981 Voyager expedition,
this exciting book looks at how Pluto was discovered and explored,
and how the pursuit of knowledge about this distant planet has
revolutionized the entire field.

Clyde Tombaugh & the Search for Planet X by Margaret
K. Wetterer, Laurie A. Caple(Illustrator). This book for
children shares the adventures of Clyde Tombaugh from his early
years making telescopes on the family farm to his work in discovering
the mysterious Planet X in 1930, which was soon named Pluto.

Clyde Tombaugh
- Articles on the man who discovered Pluto, from New Mexico State
University.

New Horizons Mission
- Find details on the current mission to Pluto which is on its
way now. Thanks to NASA JPL for the photo of the New Horizons
launch shown in this article.

International Astronomical Union
- Founded in 1919, this respected body has over 8,700 individual
members in 66 adhering countries. It is the internationally recognized
authority for assigning designations to celestial bodies and
any surface features on them.

Copyright 1999 - 2015 by John E. Shepler. Linking to this article
is welcome, but no online republication is permitted. Print media
republication rights are available at reasonable rates. Contact
me at: John (at) JohnShepler.com